This invention relates to continuous wave operation of plasma excitation and recombination lasers.
In Applied Physics Letters, Vol. 36, No. 8, page 615 (1980), W. T. Silfvast, L. H. Szeto and O. R. Wood II describe a new electric discharge device developed for producing laser action in the atomic spectra of a number of metal vapors by a segmented plasma excitation and recombination (SPER) mechanism. This laser includes a number of narrow metal strips (of the lasing species) positioned end-to-end on an insulating substrate in such a way as to leave a small gap between each pair of adjacent strips. The strips are positioned in a laser cavity containing either a buffer gas (preferably) or a vacuum and typically are 1 mm thick, 2 mm wide, and 10 mm long (hereinafter "bulk electrodes"). When a high-voltage, high-current pulse is applied to the end strips of this arrangement, a high-density metal-vapor ion plasma is formed in each gap. Once formed, these plasmas (consisting primarily of vaporized strip material) expand hemispherically, cool in the presence of the background gas (e.g., helium) at low pressure and recombine. Using this laser configuration, pulsed laser action was observed in the near infrared at more than seventy wavelengths between 0.29 and 3.95 .mu.m in 11 elements (Ag, Bi, C, Ca, Cd, Cu, In, Mg, Pb, Sn, Zn), 3 of which (Mg, Zn, In) had not been observed to oscillate in their neutral spectrum before. Some of these results are reported in the aforementioned APL article; others are reported by W. T. Silfvast et al in Applied Physics Letters, Vol. 39, No. 3, page 212 (1981) and in Optics Letters, Vol. 7, No. 1, page 34 (1982).
The SPER laser is simple to construct, can be easily scaled in length and volume, has been shown to be capable of long life, and has the potential for high efficiency. It is the subject matter of copending applications Ser. Nos. 82,308 and 367,092, filed on Oct. 5, 1979 and Apr. 9, 1982, now U.S. Pat. Nos. 4,336,506 and 4,395,770 respectively. Both patents are assigned to the assignee hereof.
Lasing action in a SPER laser is not observed with equal facility with all metals, even at high pressure of the background gas. A figure of merit, M (0&lt;M.ltoreq.1), can be derived which defines the relative ease of achieving lasing action in a metal vapor. M is defined as follows: EQU M=1/kc.rho.T.sup.2 ( 1)
where k is the thermal conductivity of the metal, c is the specific heat of the metal, .rho. is the density of the metal, and T is the absolute temperature of the surface of the metal electrode. Experimentally, metals with M.about.1, such as Cd and Na, have been found to easily produce segmented metal vapor plasmas necessary for lasing action in SPER lasers at low background gas pressures (e.g., 1-10 Torr), whereas metals with M&lt;&lt;1, such as Li, Al, Ca, and Cu, do not even produce segmented plasmas. With these metals as the background, the pressure is reduced, the discharge current is carried by a discharge in the background gas between non-adjacent electrodes, effectively shorting out the intervening metal-vapor arcs, reducing the number of metal vapor plasmas and, hence, lowering the net gain.
In another copending application Ser. No. 367,216 also filed on Apr. 9, 1982, now U.S. Pat. No. 4,441,189, and assigned to the assignee hereof, we describe how segmented metal vapor plasma discharges and pulsed lasing action in SPER devices can be achieved, even at relatively low background gas pressures, with metal electrodes of materials having M&lt;&lt;1 provided that the metal strips constitute foil electrodes. These electrodes are sufficiently thinner (typically about 10 times thinner) than bulk electrodes so that discharges occur only between adjacent electrodes, thereby eliminating the short circuiting problem associated with bulk electrodes. Using this foil electrode SPER configuration, we have achieved pulsed laser action in four metals (Li, Al, Ca, and Cu) in which laser oscillation was not possible using bulk electrodes and low pressures. As a result, we observed recombination laser action on 30 transitions with oscillating wavelengths ranging from 569.6 nm to 5460 nm. Twenty-eight of these transitions had not previously been made to undergo laser oscillation by any excitation means. In addition, we observed segmented vapor plasma discharges in a SPER device with Ni foil electrodes.
In all of our prior work, however, the SPER lasers were operated in a pulsed mode; that is, the excitation means applied a relatively short duration (e.g., 5 msec) electrical pulse. Significantly longer duration electrical signals suitable for continuous wave operation would have generated excessive heat in the electrodes, ultimately causing them to melt. Had the electrodes been so damaged, of course, laser operation would no longer have been possible.